This invention relates to circuits for driving gas discharge lamps, and particularly, though not exclusively, to circuits for driving fluorescent lamps.
In a typical prior art circuit for driving a plurality of fluorescent lamps, the lamps are driven from a high-frequency resonant circuit powered from a DC power source via an inverter. The lamps are typically coupled to the output of the resonant circuit via a transformer, and filaments of the lamps are provided with heating current from small individual windings on the output-coupling transformer.
It is known in prior art fluorescent lamp driving circuits to power-up the circuit by applying to the output-coupling transformer a voltage which ramps towards a level at which the lamps strike. Such a prior art circuit offers some advantage over a circuit in which the output-coupling transformer voltage instantly achieves a striking level. If the striking voltage is applied between the filaments of the lamps before the filaments have been heated sufficiently, the life of the lamps will be considerably shortened. Such premature application of the striking voltage causes the material of the insufficiently heated filaments to sputter as the lamp strikes, damaging the filaments. By allowing the output-coupling transformer voltage to ramp, i.e. continuously increase, towards a striking level, the filaments are to some extent pre-heated before the voltage reaches a striking level.
However, such a ramping voltage circuit does not provide optimum pre-heating of the filaments, since the filament pre-heating current (being proportional to the output-coupling transformer voltage) ramps from a low to a high value rather than remaining at a steady, optimum level. Also such a ramping voltage circuit requires an increased level of complexity (and therefore, typically, cost) to generate and control the ramp voltage.